Đáp án:
$\begin{array}{l}
a)A = 2\sqrt {48} - 6\sqrt {\dfrac{1}{3}} - \dfrac{{2\sqrt 3 + \sqrt {21} }}{{2 + \sqrt 7 }}\\
= 2.4\sqrt 3 - 6.\dfrac{{\sqrt 3 }}{3} - \dfrac{{\sqrt 3 \left( {2 + \sqrt 7 } \right)}}{{2 + \sqrt 7 }}\\
= 8\sqrt 3 - 2\sqrt 3 - \sqrt 3 \\
= 5\sqrt 3 \\
b)B = \sqrt {{{\left( {2 - \sqrt 5 } \right)}^2}} - \sqrt {6 - 2\sqrt 5 } \\
= \sqrt 5 - 2 - \sqrt {{{\left( {\sqrt 5 - 1} \right)}^2}} \\
= \sqrt 5 - 2 - \left( {\sqrt 5 - 1} \right)\\
= - 1
\end{array}$
